The present invention relates to a method of separating nitrogen from air in which compressed and purified air is separated in a single column and refrigeration is added by a an external liquid stream. More particularly, the present invention relates to such a method in which the column is refluxed by condensing tower overhead against vaporizing liquid column bottoms to produce an oxygen rich waste stream. Even more particularly, the present invention relates to such a method in which the air is purified in adsorbent beds that are regenerated and pressurized in part by the oxygen rich waste stream.
Air is separated by a low temperature rectification process utilizing a single column known as a nitrogen generator. The air is compressed, purified and cooled to near its dewpoint and is then introduced into the distillation column to produce a nitrogen rich tower overhead and an oxygen rich liquid column bottoms. A stream of the column bottoms is expanded and then used as a coolant to condense tower overhead that is in turn used to reflux the column. Refrigeration is added by injecting a liquid stream into the process in order to compensate for warm end losses and heat leakage into the process. Commonly, the refrigeration is added by way of a liquid nitrogen stream injected into the distillation column. An air separation plant utilizing an external source of liquid to provide refrigeration is known in the air as a "liquid assist plant".
In any air separation plant it is necessary for the air to be prepurified so as to remove carbon dioxide and moisture that would otherwise solidify within piping and heat exchange passages within the plant. In air separation plants designed to produce high purity products, it is also necessary to remove other impurities such as carbon monoxide and hydrocarbons.
The necessary prepurification can be effectuated by known prepurification units used in connection with liquid assist plants such as described above. Prepurification units comprise adsorbent beds to adsorb the aforementioned impurities from the incoming air. In order that the plant can continuously be operated, the adsorbent beds are utilized in continuous cycles in which one adsorbent bed functions in an on-line capacity to adsorb the impurities from the air with one or more other adsorbent beds are in an off-line state being regenerated. One major cycle is referred to in the art as pressure swing adsorption in which the beds adsorb at an operating pressure of the plant that is set by the main air compressor and are then regenerated at a lower pressure at which the adsorbed impurities desorb and are expelled from the unit.
Typically the regeneration in a pressure swing adsorption cycle is effected by first venting an adsorbent bed to atmosphere so that the adsorbed impurities desorb from the bed and flow towards the bed inlet. Thereafter, the adsorbent bed is purged with a stream of nitrogen to further desorb the adsorbed impurities. The adsorbent bed is then repressurized with part of the compressed air that has been compressed by the main air compressor. The use of part of the air to be rectified not only represents an increase in the required capacity of the compressor but also an additional energy expenditure beyond that required for the distillation itself.
As will be discussed, the present invention provides a method of separating nitrogen from air in which a liquid assist plant is operated in connection with a pressure swing adsorption, prepurification unit in a more energy efficient manner than the prior art and such that inherently decreases compressor capacity requirements.